The use of polyepoxides, known as epoxy resins, in coating compositions has been well established for years. Such coatings have a spectrum of physical and chemical properties that make them well adapted for a variety of applications.
Polyepoxides are cured into a cross-linked, thermoset state by a variety of curing agent systems. The choice of curing agent system is generally a compromise between competing factors. It is usually desirable to have the coating cure as rapidly as possible. However, such a system will also cause premature gelation or at least an unacceptable viscosity increase in the composition before application to the substrate. Thus, either small amounts of composition can only be made prior to deposition of the coating or some sacrifice in cure rate has to be made to prolong the workable life of the composition, sometimes called the pot life of the composition.
One of the solutions to that problem has been to employ a curing agent system that is relatively stable at ambient conditions but cures rapidly at elevated temperatures. That technique is not available for all applications, however.
A method of providing water-borne epoxy paints which cure well even under unfavorable conditions of high humidity and cool temperatures was described in the copending application of McFadden et al., U.S. Ser. No. 103,330, filed Dec. 14, 1979 now U.S. Pat. No. 4,272,621. That teaching involved partially neutralizing a vinyl interpolymer having pendant aminoester group with a mixture of aqueous hydrochloric and hydrobromic acids; the whole mixture being a single liquid phase dissolved in a glycol ether solvent.
Such solvent-borne resins can be mixed with a polyepoxide, such as a diglycidyl ether of bisphenol A, the whole diluted with water, and, with or without pigmentation, a paint film may be cast from this mixture which cures to a hard, tenacious protective coating for steel, wood, glass and other substrates.
U.S. Pat. Nos. 3,719,629, 3,634,372 and 3,538,185 describe the use of both unneutralized and neutralized aminomethacrylate resins in organic solvents used in conjunction with bisphenol A diglycidyl ether resins to produce epoxy coatings having superior resistance to chalking and yellowing by sunlight.
The use of so-called "ketimines" in epoxy coating systems is well-known and widely practiced ("Handbook of Epoxy Resins", Lee and Neville, pp. 7-25 and 7-26, 1967, and "Epoxy Resins Chemistry and Technology", May and Tanaka, pp. 469-470, 1973). In general, ketimines are produced in a separate chemical process in which a polyamine curing agent and a ketone are heated in the presence of an acid catalyst, and water of reaction is removed by distillation, flash-evaporation, or other technique. Such operations are costly and involve boiling and condensing large quantities of highly flammable organic liquid. Alternatively, the ketone and amine may be packaged together as one part of the two-part amine/epoxy coating. In this case, much of the desirable effect of using ketimines, to wit, long pot life with good cure rate is lost. The retardation of gelation in the paint can is minimal with substantial viscosity increases in four to five hours.